The present invention relates to sensing of an external influence with high precision and sensitivity that makes use of continuous magnification within each sensing cell to significantly magnify an electrical off-balance resulting from exposure to an external influence.
There are many areas of technology that require sensors to perform a measurement of the magnitude of an external influence. The influence may be electromagnetic, such as gamma rays, X-rays, ultraviolet, infrared, visual light, or radio frequency waves; the influence may be electrostatic or magnetic. The influence may be vibrational, such as ultrasound; or thermal; or resulting from beamed particles. The influence may take other forms less-well developed in current technology, such as force-derived influences (e.g., pressure or stress variations, tactile images, etc.), and chemical variations (e.g., olfactory and gustatory). In the remainder of this specification and in the claims, the term "image" will be used to refer to any form of external influence, without restriction to merely optical influences.
In image sensing arrays designed for each of the above applications, a common element is the electronic structure of the array and the sensing cells. The array may employ an orthogonal grid of conductive lines to address and sense individual sensing cells, wherein an amount of electrical charge has been developed as a result of exposure to the image. The sensing cells have means to develop a charge in response to the external influence.
For economic reasons, it is desirable to make each sensing cell as small as possible, and often to include many thousands of such cells within each array. It is generally true that the smaller each cell becomes, the less an amount of charge that may be developed in response to the image. On the other hand, the larger the array becomes--that is, the larger the number of cells in the array--the more difficult it becomes to detect the charge developed within a given cell due to the capacitance of the sensing lines, capacitive coupling between cells, and to noise introduced during the sensing process.
Heretofore, the majority of image sensors have been designed with image sensing cells having charge storage means to collect the charges resulting from the exposure to the image, and means to send the charges to external sensing means upon actuation of a control pulse. Because they contain no means for amplifying the charge within the cell, such image sensors are limited in the degree to which they can sense low image intensities.
Several image sensors in the prior art represent an attempt to improve the sensitivity of the array by including means within each cell to amplify the detected charge prior to placing the signal on the sensing lines of the array. The most common is to develop the charge on a capacitor which is further connected to the control lead of a current control element such as a bipolar transistor or FET. Although amplification is obtained through the amplifying action of the current control element, the degree of amplification is substantially dependent on fixed properties of the element. For example, there is a maximum amplification that can be obtained from a single transistor (or cascaded series of transistors, in case several are utilized within each cell). In addition, although the degree of amplification may be electronically controllable to some degree by controlling the other leads of the transistor, the range of available amplifications is limited.
Another drawback of conventional image sensors is due to the fact that the amount of charge modification effected by an image within each cell is generally proportional to the intensity of the image, whereas greatest sensitivity (amplification) is needed for low image intensities and lesser sensitivity for greater image intensities. Ideally, the signal leaving the cell should be logarithmically related to the image intensity, for then a greater dynamic range of image intensities will produce useful signals than in the case of a proportional relation. For example, if useful output voltages of an image cell can lie between 0.2 and 1 volt, a dynamic factor of 5 is available to a proportional sensing cell, while a dynamic range of 10,000 is available to a logarithmic sensing cell producing 0.2 volt per decade of image intensity. In the majority of prior art image sensors, both those without amplification means and those including proportional amplification means, the signal sent from each image sensing cell is proportional to the image intensity and therefore of limited dynamic range.
It is also known that the switching signals introduced into the array to perform the necessary addressing of the image cells may cause considerable noise in the detected signal. One method for reducing the effect of switching noise in the addressing of an array, which is commonly used in memory arrays, is to provide complementary sense lines to each cell, such that the signal developed within the cell will be placed in complementary fashion on the sense lines while any switching noises will occur substantially in common on the lines. By subtracting the signal on one line from its complementary line, much of the switching noise may be eliminated without diminution of the signal.
On the other hand, when employing complementary structure, there are often minor imbalances in the components manufactured for each of the complementary channels. In the case of extreme magnification, these minor differences may become magnified to cause considerable distortion of the developed signal.